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288           MOBPHOLOGI AND EVOLUTION
be a few small retrocalcarine grooves. And a gyms separates it from the internal parieto-occipital sulcus. The collateral sulcus is well marked, and the calloso-marginal sulcus (C-1I.S.) appears for a short distance on the outer surface of the hemisphere.
The cerebellum differs from that in Hylobates. The bulk is relatively greater, the paraflocculus is much reduced, and there is no petrosal lobule. The relative developments of the flocculus and paraflocculus vary, however, in different brains. The lateral parts of the trapezium are exposed.
THE BHAIN is THE CHIMPANZEE (fig* 51).
The brain is larger than that in the Orang, and smaller than that in the Gorilla. The characters are very variable, and it is only by examining many specimens that one can come to the conclusion that the Gorilla's brain most approaches that of Man. The brain is ovoid in shape, and the inferior surfaces of the frontal lobes are rostrated. The cerebrum overlaps the cerebellum to a variable extent.
The central sulcus (C.S.) has genua of varying degrees of acuteness. It may not reach the great longitudinal fissure.
In front of the central sulcus there lie superior (S.P.S.) and inferior precentral (LP.S.) sulci, which may come very close together. The former unites with one of the superior frontal sulci (S.F.S.) to make a triradius, and the latter does the same with the long inferior frontal sulcus (I.F.S.). Middle frontal sulci (MJ\S.) may be well marked. Sometimes the inferior precentral sulcus joins the central sulcus: and the anterior extremity